Compared to a thin film transistor liquid crystal display (TFT-LCD) which is the mainstream display technique in nowadays, an organic light emitting diode (OLED) has advantages such as wide viewing angle, high brightness, high contrast, low power consumption, thinner thickness and lighter weight, and becomes the focus of attention in the field of flat panel display technology.
Driving methods of the organic light emitting diode displays are classified into two types: passive matrix type and active matrix type. Compared to a passive matrix type organic light emitting diode display, an active matrix type organic light emitting diode display has advantages such as ability to display large amount of information, low power consumption, long service life of devices, high contrast of picture and the like.
In an organic light emitting diode display, a plurality of pixel unit driving circuits are provided. Each of the pixel unit driving circuits is connected with a driving power supply, thus together forming a display driving circuit for display.
FIG. 1 is a schematic diagram of a pixel unit driving circuit of an active matrix type organic light emitting diode display in the prior art, and as shown in FIG. 1, the pixel unit driving circuit includes: a switching transistor M1, a driving transistor M2, a storage capacitor C and a light emitting device OLED, wherein, a gate of the driving transistor M2 is connected to a drain of the switching transistor M1, a source of the driving transistor M2 is connected to a driving power supply 1 (sources of driving transistors M2 of a plurality of pixel unit driving circuits are connected to the same driving power supply 1), a drain of the driving transistor M2 is connected to the light emitting device OLED, and when the switching transistor M1 is turned on under the control of a scanning signal Vscan, a data voltage Vdata is transferred to the gate of the driving transistor M2 through the switching transistor M1. In the meanwhile, the driving power supply 1 supplies a driving voltage VDD to the source of the driving transistor M2. Gate-source voltage of the driving transistor M2 is Vgs, which determines magnitude of a driving current flowing through the driving transistor M2, and the driving current is used for driving the light emitting device OLED to generate stable light. The function of the storage capacitor C is to maintain the stability of gate voltage of the driving transistor M2 during one frame time.
When the light emitting device OLED emits light, a voltage drop VD1 generated by the light emitting device OLED, a voltage drop VDS on a load current path (drain-source path) of the driving transistor M2 and the driving voltage VDD generated by the driving power supply 1 satisfy the following relationship: VDD=VDS+VD1.
FIG. 2 is a schematic circuit diagram of the driving power supply in FIG. 1, and as shown in FIG. 2, the driving power supply includes: a boost module, one terminal of which is connected to an initial voltage input terminal and the other terminal of which is connected to the driving transistor M2 in the pixel unit driving circuit. The boost module is used for boosting an initial voltage VCC input from the initial voltage input terminal to obtain the driving voltage VDD, and outputting the driving voltage VDD to the driving transistor M2. The boost module includes: a boost chip 2, an energy-storage inductor L, a first switching tube T1, a Schottky diode D, a first resistor RA, a second resistor RB and a first filter capacitor C1, wherein, one terminal of the energy-storage inductor L is connected to the initial voltage input terminal, the other terminal of the energy-storage inductor L is connected to a first terminal of the Schottky diode D and a second electrode of the first switching tube T1, an input terminal of the boost chip 2 is connected to the initial voltage input terminal, a feedback terminal of the boost chip 2 is connected to the first resistor RA and the second resistor RB, a control terminal of the boost chip 2 is connected to a gate of the first switching tube T1, the first terminal of the Schottky diode D is connected to the second electrode of the first switching tube T1, and a second terminal of the Schottky diode D is connected to the first filter capacitor C1.
Boosting voltage can be achieved by controlling a field effect transistor (not shown) integrated inside the boost chip 2 to be turned on or off. Specifically, when the field effect transistor integrated inside the boost chip 2 is turned on, the Schottky diode D is turned off reversely, current in the energy-storage inductor L increases constantly, and the energy-storage inductor L stores energy; when the field effect transistor integrated inside the boost chip 2 is turned off, the energy-storage inductor L outputs through the Schottky diode D, thus accomplishing energy transfer. The feedback terminal of the boost chip 2 controls turn-on time and turn-off time of the integrated field effect transistor according to voltage across the second resistor RB, thereby controlling magnitude of the driving voltage VDD output from the boost module.
FIG. 3 is a diagram illustrating the driving principle of an active matrix type organic light emitting diode display in the prior art, and FIG. 4 is graph illustrating relationship between brightnesses of red, green and blue organic electroluminescent devices and voltage drops thereof As shown in FIGS. 3 and 4, the organic light emitting diode display includes pixel units of three different colors: red (R), green (G) and blue (B), wherein, a red organic electroluminescent device OLEDR is provided inside a red pixel unit, a green organic electroluminescent device OLEDG is provided inside a green pixel unit, a blue organic electroluminescent device OLEDB is provided inside a blue pixel unit, and all of the pixel units are driven by the same driving voltage VDD (magnitude of the driving voltage VDD satisfy the condition that the blue organic electroluminescent device OLEDB can be driven when reaching its maximum brightness).
Referring to FIG. 4, since light emitting layers of the organic electroluminescent devices of three different colors are made of different semiconductor materials, the organic electroluminescent devices of three different colors generate different voltage drops when they have the same brightness. The voltage drop generated by the blue organic electroluminescent device OLEDB is the largest, the voltage drop generated by the red organic electroluminescent device OLEDR follows, and the voltage drop generated by the green organic electroluminescent device OLEDG is the smallest. Here, since all of the pixel units are driven by the same driving voltage VDD, gate-source voltages of the driving transistors inside the red and green pixel units will be large. However, loading large voltage on the driving transistor will not only heat the driving transistor and further impact service life of the driving transistor, but also result in large power consumption of the display driving circuit.